The invention relates to a condensation dryer having a drying chamber for the items to be dried, a process-air circuit, a first fan in the process-air circuit, a heat pump in which a coolant circulates and that has an evaporator, a compressor, a condenser, and a throttle, and further having a temperature sensor for measuring a temperature of the coolant, and a controller, and also to a preferred method for operating the dryer.
A condensation dryer of said kind and a method for operating it proceed from DE 40 23 000 C2.
In a condensation dryer, air (what is termed process air) is ducted by a fan across a heater into a drum as a drying chamber containing damp laundry items. The hot air absorbs moisture from the laundry items requiring to be dried. Having passed through the drum, the then moist process air is ducted into a heat exchanger upstream of which as a rule a lint filter is connected. The moist process air is cooled in said heat exchanger (for example an air-air heat exchanger or a heat pump's heat sink) so that the water contained in the moist process air condenses. The condensed water is then generally collected in a suitable container and the cooled and dried air ducted back to the heater (which may be a heat pump's heat source) and then to the drum.
That drying process is in certain circumstances very energy-intensive because the cooling-air current heated in the heat exchanger as the process air is cooled can in energy terms be lost to the process. That energy loss can be significantly reduced by employing a heat pump. In the case of a condensation dryer fitted with a heat pump the warm, moisture-laden process air is cooled substantially in a heat sink of the heat pump, where the heat extracted from the process air is used for, for example, evaporating a coolant employed in the heat-pump circuit. The heat absorbed in the heat sink is transported inside the heat pump to the heat source and there given off again—possibly at a temperature raised above that at the heat sink. In a heat pump, which operates with a coolant as the heat-transporting means, with the coolant being evaporated in the heat sink and condensed in the heat source, via a compressor the evaporated, gaseous coolant reaches the heat source, which can here be designated a condenser, where, owing to the gaseous coolant's being condensed, heat is released that is used for heating the process air before it enters the drum. The condensed coolant finally flows back to the evaporator through a throttle; the throttle serves to reduce the internal pressure in the coolant so it can evaporate in the evaporator with heat again being absorbed. The heat pump that is operated in such a way with a circulating coolant is known also as a “compressor heat pump”. Other heat pump designs are also known.
DE 40 23 000 C2 discloses a laundry dryer that has a heat pump and arranged in which in the process-air channel between the condenser and evaporator is an incoming-air orifice that can be sealed with a controllable sealing device.
WO 2008/086933 A1 describes a condensation dryer having a drying chamber, a process-air circuit having a heater for heating the process air and a fan, an air-air heat exchanger and a heat-pump circuit having an evaporator, a compressor, and a condenser. Located in the heat-pump circuit between the condenser and evaporator is an additional heat exchanger that is functionally coupled to the air-air heat exchanger. The temperature of the heat pump's coolant, particularly in the condenser, is kept within the permissible range via the heat pump's controller and the additional heat exchanger.
DE 40 34 274 A1 describes a laundry dryer and a method for monitoring the temperature therein, with the laundry dryer having a thermostat device connected to a heater for air and provided for registering the temperature of the air current, which device is set up for disconnecting the heater if an upper temperature value is exceeded and for connecting the heater again when the temperature falls below a lower value, and further having a monitoring circuit connected to the thermostat device for producing a signal, dependent on the disconnections, for an indicator unit. The monitoring circuit has a counter circuit for counting the heater disconnections having occurred during a drying process and is connected to the indicator unit. A decoding circuit can produce a fault signal when a specific number of disconnections has occurred. For example the laundry dryer's heater will be disconnected if the number of heater disconnections exceeds a reference value and a display unit will simultaneously be driven via which users are able to recognize or hear that they need to clean the lint filter and/or the condenser, or that the fan has suffered an outage.
DE 197 28 197 A1 discloses a method for detecting unacceptable operating conditions in a laundry dryer as well as a laundry dryer having the detection method of such kind. The aim of the method is to enable separate or joint recording of different operating conditions of too high temperature that originate in different regions. The temperature is recorded periodically in the supply-air current above a supply-air heater and in front of the laundry drum, a difference value or gradient is created from two successively recorded values, said difference value (gradient) is compared with a preset difference value (gradient), with—if the newly created difference value is greater in absolute terms than the preset difference value—a counting value being raised by a step, said value being compared with a preset value, and—if the current value is greater than the preset value—the laundry-dryer heater being switched off and/or an operating-condition display activated.
The traditionally employed air-air heat exchanger—operated in crossover or counterflow mode—and the electric heater are generally completely replaced with a heat pump. Compared with a dryer having an air-air heat exchanger and a resistance heater it is possible thereby to achieve a 20-50% reduction in the energy required for a drying process.
A compressor-heat pump as a rule operates optimally within specific temperature ranges in the evaporator and the condenser. What is problematic about using a compressor-heat pump in the condensation dryer is the usually high temperature in the condenser, which for process reasons can result in its no longer being possible to condense or fully condense the coolant; the compressor will then have to be switched off and/or a substantial impairment in the heat pump's effectiveness will have to be accepted. That problem is even worse when the compressor is supported by an additional heater in the process-air circuit to achieve faster heating of the process air and hence shorter drying times. Moreover, the circulating process air can be impeded by soiled air paths. That can likewise cause the temperature of the coolant to rise. Operating states of such kind can result in damage to the heat pump or other parts of the dryer and so are impermissible.
In a conventional dryer an impermissible operating state, for example a reduced circulation of the process air (reduction in air performance) is ascertained by registering a temperature in the process-air current above a heater for the process air and in front of the drying chamber at regular intervals and forming from in each two successively registered values a difference value corresponding to a time gradient. That information generally does not have to be available in that form in the case of a dryer fitted with a heat pump (a heat-pump dryer). For example in a heat-pump dryer the heat pump is frequently sited further from the drying chamber than is the heater in a conventional condensation dryer. In any event, detecting an impermissible operating state in a condensation dryer fitted with a heat pump can in that way be done only imprecisely.